pdnsutil: possibly helpful backend-cmd help

[miodvallat]

Short description

At the moment, if one invokes pdnsutil backend-cmd $backendname with no actual commands to perform, you get a syntax error and the synopsis of pdnsutil backend-cmd.

However, the LMDB backend has code to display a short but much more useful help message in this case.

So let pdnsutil pass no commands to the backends, and let the backends handle that - either doing nothing or giving some information.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document, including the AI Policy, and added a "Signed-off-by" to my commits
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)